1. Field of the Invention
The present invention relates to a processes for producing polyethylene resin particles. In greater detail, the present invention relates to a process for producing polyethylene resin particles suitable for producing foamable polyethylene resin particles having good molding properties which can be prefoamed to a low bulk density and can form moldings having good rigidity, from which suitable articles can be produced by adding a blowing agent to the polyethylene resin particles to form foamable polyethylene resin particles, heating the particles to produce prefoamed particles, charging the pre-foamed particles in a mold cavity which permits the escape of gases but retains the foamable polyethylene resin articles, and heating them; and to a process for producing foamable polyethylene resin particles containing a polystyrene resin uniformly dispersed therein which comprises adding a blowing agent to the resulting resin particles.
2. Description of the Prior Art
In order to produce foamable polyethylene resin particles which can be pre-foamed to a low bulk density, it is necessary to impregnate a blowing agent consisting of volatile aliphatic hydrocarbons such as n-butane, iso-butane, n-pentene, iso-pentane and dichlorodifluoromethane into the polyethylene resin particles. However, although the particles containing the blowing agent can be foamed to a low bulk density just after production thereof, it is necessary to leave the particles for a definite period because the cells produced just after production are very unstable, i.e., because the polyethylene resin has a high gas permeability. Consequently, there is a disadvantage in that particles having high bulk density are actually produced if the particles are allowed to i.e., because the blowing agent disappears on standing. At the same time methods for producing foamable polystyrene resin particles, for example, a method which comprises suspending polystyrene resin particles in an aqueous medium and adding a small amount of a solvent which dissolves the resin particles and adding a blowing agent to the resulting suspension to impregnate the blowing agent in the resin particles, and a method which comprises blending polystyrene resin particles and a blowing agent in an extruder, molding by extruding as thin strands, quenching immediately and pelletizing the strands, cannot be applied to production of the foamable polyethylene resin particles. Even if such methods could be so adopted, the molding conditions are very severe (the viscoelasticity of the polyethylene at foaming is less than that of the polystyrene resin), and consequently it would be difficult to produce preferred articles and moldings.
A process for cross-linking the polyethylene resin has been suggested as a means for preventing the disappearance of the blowing agent (volatile aliphatic hydrocarbons) included in the foamable polyethylene resin particles and as a means for adjusting the viscoelasticity at foaming. Namely, a method is known which comprises impregnating the blowing agent using various techniques and then cross-linking the polyethylene resin by applying radiant rays before the blowing agent disappears and therefore is lost, and a method is described in Japanese Pat. No. 32622/70 which comprises carrying out impregnation of the blowing agent simultaneously with the cross-linking in the suspension. According to the former method, the cost of the equipment is high, although cross-linking can be carried out in a short period of time. Therefore, this method is not economically preferred because the foamable polyethylene resin particles produced are expensive. In the latter method, since the cross-linking and the impregnation are carried out at the same time, the impregnation of the blowing agent is, of course, carried out at the cross-linking temperature. Accordingly, where n-propane, dichlorodifluoromethane and chlorodifluoromethane, which are always gaseous, are used as the blowing agent, the pressure abnormally increases. Thus, if the cross-linking and the impregnation are carried out under such conditions, the polyethylene resin particles are flattened by such high pressures and good foamable polyethylene resin particles cannot be obtained. Further, in order to carry out the method at such high pressures, it is necessary to use a high-pressure high-temperature reactor.
Further, the method descried in Japanese Pat. No. 32623/70 has been suggested. According to this method, since the polystyrene resin is uniformly included in the polyethylene resin particles, disappearance of the blowing agent can be prevented and articles having somewhat improved rigidity can be obtained. However, since cross-linking of the polyethylene resin, polymerization of styrene and impregnation of the blowing agent are carried out at the same time, the blowing agent causes a decrease in the viscosity of the mixture and consequently a long period of time is required for polymerization. In order to prevent these defects, the quantity of the styrene monomer used is limited to below 20% by weight based on the polyethylene resin particles. Furthermore, since the polymerization of the styrene monomer is carried out at a temperature below the cross-linking temperature, the polyethylene resin particles containing polystyrene resin having a low degree of polymerization. Consequently, when pre-foamed particles are produced therefrom by heating, cells in the pre-foamed particles become nonuniform or pre-foamed particles having an excellent molding property cannot be obtained because the second foaming ability thereof is decreased. Still more, this method has the disadvantage that articles having excellent rigidity can not be obtained. Moreover, in a case of using a gaseous blowing agent such as n-propane, n-butane or dichlorotetrafluoroethane, it is necessary to use a high-pressure autoclave because the reaction system is under a high pressure.